In this project we propose to study the mechanism of degradation of components of the myocardial cell in the steady state and under conditions of work overload. The major objectives are: 1. Comparison of degradation rates of specific myocardial proteins. In this approach we will utilize our recently developed techniques which allow us to determine the true half-lives of myocardial proteins. (The kinetic analysis of incorporation data is based on amino acid radioactivity in tRNA and in electrophoretically purified proteins.) The following proteins will be compared: the myofibrillar proteins; myosin (heavy chains, HC; light chains, LC1, LC2 alpha-actinin, actin, tropomyosin, troponin (TN-1, TN-C, TN-T); sarcoplasmic reticulum proteins: ATPase, calsequestrin high affinity protein, low molecular weight acidic protein. 2. Study of factors determining the rate of protein degradation. The myofibrillar proteins of known half-life will be compared with respect to susceptibility to proteolytic digestion by calcium-activated neutral protease (CaAP) cathespin D and trypsin. The kinetics of digestion of actin and of troponin subunits will be studied in detail to elucidate their respective resistance and sensitivity to protease attack. 3. Mechanism of myofibrillar assembly. Our objective is to determine whether the process of myofibrillar turnover is a random or directional process. We will study the labeling kinetics of myosin heavy chains in easily released and core myofibrillar filaments. The process of disassembly will be studied in tissue culture systems of cell aggregates. The process of filamental assembly can be controlled at will by manipulation of culturing conditions. 4. Characterization of myosin synthesized in work-overloaded myocardium. We will compare the rates of synthesis of HC and LCs in pressure overloaded myocardium. The respective chains will also be compared with respect to cyanogen bromide cleavage products and immunological properties. The phosphorylation of myosin light chains will be compared under conditions leading to opposite changes of ATPase (i.e., increase in physiologically overloaded and decrease in pathologically overloaded myocardium).